1. Field of the Invention
The present invention concerns an aluminum alloy of excellent machinability suitable, for example, to machine parts which often undergo machining fabrications in the course of manufacture.
2. Description of Related Art
Among aluminum alloys, not heat treated alloys including 3000 series Al--Mn alloys have medium mechanical performances, are excellent in corrosion resistance and cold forgeability and can be formed at a low cost. They have generally been used, for example, as machine parts, in which they undergo machining or drilling fabrication after cold forging into final products. However, it is difficult to use the alloys of this series to machine parts requiring complicated machining or drilling since chips formed during machining are difficult to remove and deteriorate machinability.
Further, among aluminum alloys, not heat treated alloys including 5000 series Al--Mg alloys have medium mechanical performance (somewhat higher strength level than 3000 series), are excellent in corrosion resistance and cold workability and can be fabricated at a reduced cost. They have generally been used, for example, to manufacture optical instruments such as cylindrical members of cameras and microscopes and other machine parts, in which they generally undergo machining or drilling fabrication after cold forging into final products. However, it is difficult to use the alloys of this series to machine parts requiring complicated machining or drilling fabrication since chips formed during machining are difficult to remove and deteriorate machinability.
On the other hand, existent aluminum alloys of high machinability contain low melting metals such as Pb, Bi and Sn as effective addition elements as typically represented by AA6262 alloy (Si: 0.4-0.8 mass %, Mg: 0.8-1.2 mass %, Cu: 0.15-0.4 mass %, Pb: 0.4-0.7 mass %, Bi: 0.4-0.7 mass % and the balance of Al) in the field of ductile material (refer to Japanese Patent Laid-Open Sho 54-143714, Japanese Patent Laid-Open Hei 3-39442). Such low melting metals are barely solid-solubilized in aluminum and cause granular micro segregation in the aluminum alloy. The low melting metal grains are melted by the heat of fabrication generated upon machining fabrication and act to remove the chips and improve the machinability of the aluminum alloys.
The AA6262 alloys are heat treated type aluminum alloys employed as the raw material for machine parts which undergo machining fabrication, particularly, drilling in the course of manufacture. For example, they are used as a material for the housing of an anti-skid brake system of an automobile. It is expected that the effect of improving the machinability by the addition of the low melting metals such as Pb, Bi and Sn can be obtained not only in the heat treated alloys but also in the not heated treated alloys (refer to Japanese Patent Laid-Open Hei 3-39442 described above).
However, although the addition of the low melting metals to the aluminum alloys can improve the machinability this lowers the corrosion resistance and causes hot shortness by the low melting metals and it is necessary to employ sufficient care in working the alloys. Further, only alloys containing Pb and Bi can be recycled as scrap, as a result their recycling performance is poor. Thus, their usefulness is limited.
Further, the machine parts are sometimes anodized at the surface to improve corrosion resistance, wear resistance or decorative effect. However, with Pb and Bi-added aluminum alloys, oxide films are not formed on regions of the surface at which Pb and Bi are exposed and this results in inhomogeneous and non-glossy anodic oxidation films.
Although not heat treated aluminum alloys not containing low melting metals and having improved machinability were proposed in Japanese Patent Laid-Open Sho 60-184658, the machinability was not sufficient as compared with the aluminum alloys containing low melting metals such as Pb, Bi and Sn.